This invention relates generally to the series capacitor connected in electric power transmission systems and more particularly to an electric discharge gap device for protecting the series capacitor against an overvoltage thereacross.
Recently the demand of electric power is rapidly increased in urban areas including the districts around cities but locations of power production are far away from the urban area as a result of the reflex of the overpopulation in cities and the land situation. On the other hand, the production of electric power has the tendency to be effected on an increasingly large scale. Thus electric transmission systems connecting locations of power production to associated areas where the electric power is demanded becomes very long in line length. Also since the transmission capacity required will become high to an unprecedented extent, it is requested to efficiently and safely supply the stabilized electric power with a low loss upon future power transmission.
Further power transmission systems generally become unstable with increases in line constants such as the line resistance, line impedance etc. and also with an increase in transmission distance. This results in a gradual decrease in a quantity of possible transmission of electric power. As a result, the series capacitor has been highlighted for the purpose of increasing the transmission capacity, improving the stability of the transmitted voltage and so on during high capacity long distance transmission.
Upon the occurrence of a fault in the system, an excessive current flows through this series capacitor to render a voltage thereacross extremely high. This leads to damage 7 the series capacitor accompanied by disabling of the power transmission. In order to protect the series capacitor against this increase in voltage, a protective gap device has been connected across the series capacitor. That protective gap device is required to rapidly protect the series capacitor against an overvoltage developed thereacross while, after the removal of the system fault, permitting the series capacitor to be rapidly re-connected in the system to accomplish to the proper purpose of the capacitor. In other words, the protective gap device preferably performs the accurate operation and has the ability to withstand a high discharge current from the capacitor while retaining the same protective capability as prior to the electric discharge even in the transient state where the series capacitor is again being connected in the system after the extinction of the particular electric arc across the gap device.
One of the conditions required for the protective gap device to fulfil the duties such as above described is to prevent discharge electrodes involved against damaging due to an arc current developed during an electric discharge as far as possible and to restrain a decrease in the discharge characteristic of the device. Also if a product of electric discharge is formed in a large amount then the discharge characteristic of the device is affected. This leads to the necessity of paying attention to prevent an electric arc from touching electrode support means, a housing surrounding electrodes, etc.
Conventional discharge gap devices commonly employed, have been designed and constructed such that the electric arc struck across the main electrodes is transferred to consumable electrodes by means of the action of an electromagnetic force due to an arc current flowing through the electrodes whereby the electric arc is prevented from remaining on the surfaces of the main electrodes at one position. Thus the electric arc tends to spread toward the housing encircling the electrodes and thereby greatly affect the housing. This has resulted in the disadvantage that conventional discharge gap devices can not be constructed into enclosed structures extremely small-sized and compact.